Single-atom platinum with asymmetric coordination environment on fully conjugated covalent organic framework for efficient electrocatalysis

Two-dimensional (2D) covalent organic frameworks (COFs) and their derivatives have been widely applied as electrocatalysts owing to their unique nanoscale pore configurations, stable periodic structures, abundant coordination sites and high surface area. This work aims to construct a non-thermodynamically stable Pt-N2 coordination active site by electrochemically modifying platinum (Pt) single atoms into a fully conjugated 2D COF as conductive agent-free and pyrolysis-free electrocatalyst for the hydrogen evolution reaction (HER). In addition to maximizing atomic utilization, single-atom catalysts with definite structures can be used to investigate catalytic mechanisms and structure-activity relationships. In this work, in-situ characterizations and theoretical calculations reveal that a nitrogen-rich graphene analogue COF not only exhibits a favorable metal-support effect for Pt, adjusting the binding energy between Pt sites to H* intermediates by forming unique Pt-N2 instead of the typical Pt-N4 coordination environment, but also enhances electron transport ability and structural stability, showing both conductivity and stability in acidic environments.


Dear Editors and Reviewers:
Thank you for your letter and comments concerning our manuscript entitled "Single-

Atom Platinum Sites with Asymmetric Coordination Environment on Fully Conjugated Covalent Organic Framework for Efficient Electrocatalysis"
(Manuscript ID: NCOMMS-23-59946).We have revised the manuscript based on your suggestions and highlighted the revisions for your review.The point-to-point responses to the reviewers' comments are listed below:

Reviewer #1:
In this work, the authors developed a conductive agent-free and pyrolysis-free method to synthesize Pt single atoms anchored on nitrogen-rich graphene analogue COF, which regulating the HER activity and stability in full pH.Systemic characterizations and DFT calculations solidly demonstrated that the electronic structure could be modified by unique Pt-N2 coordination environment.The results are interesting.Although some issues should be addressed in the revision process.

Comment 1:
What unique structure of COF compare to other 2D materials such as graphen, layered double hydroxides (LDHs) and MOFs, supporting the corrosion resistance in acidic environments?
Response 1: a.For LDH, it is a kind of hydroxide, which is alkaline.The layered structure of LDH is stabilized by hydroxyl between the layers.Structural collapse and dissolution will occur in acidic environment, hindering their further application as electrocatalysts in acidic environment.b.For most MOFs, the degree of reversibility of coordination bonds between metal nodes and ligands is much higher than that of covalent bond between ligands in covalent organic frameworks, which endows MOF higher crystallinity, but also makes MOF more vulnerable to harsh pH environments compared to COF.During the electrocatalytic process, the applied voltage damages the dynamic coordination bonds, eventually leading to the reconstruction of the overall structure (ACS Catal.2022, 12, 16, 10276-10284; ACS Energy Lett. 2019, 4, 4, 987-994).As a result, MOFs are typically used as pre-catalysts rather than catalysts in electrocatalysis.Some chemical modifications, such as pyrolysis, are necessary to stabilize the dynamic metal-ligand bonds, enabling their application in harsh pH environments.c.Graphene, as one of the most representative two-dimensional conductive network materials, exhibits stability and excellent electrical conductivity under acidic conditions, making it an ideal substrate for anchoring noble-metal atoms.However, enhancing intrinsic activity and achieving an ideal metal-support interaction for regulating the electron structure of Pt single atoms on graphene remains challenging.Moreover, due to the structural stability of graphene, achieving uniform dispersion of noble-metal single atoms often requires high temperature and pressure, increasing the uncertainty of the final catalyst structure and hindering subsequent mechanism analysis.d.In this work, the ligands of NGA-COF are linked by covalent imine bonds with high bond energy.Previous studies have confirmed the stability of these bonds in acidic conditions (Nat.Catal.2022, 5, 414-429, Small 2019, 15, 1903643).In-situ Raman was carried out to demonstrate that the structure of NGA-COF@Pt remained intact in the HER process (Fig. 5e and Supplementary Fig. 35).In addition, abundant N coordination sites among NGA-COF allow Pt single atoms to be introduced more mildly at room temperature and pressure.Therefore, NGA-COF is a more ideal substrate for noble-metal single atoms in acidic environment than graphene, LDHs, and MOF.

Comment 2:
How do you prove that synthesized NGA-COF is AA stacking structure in line 143?Is there any more intuitive evidence?b.According to nitrogen adsorption-desorption isotherms (Fig. 1d), the pore size distribution of the synthesized NGA-COF is concentrated in 11.8 Å, which is consistent with the theoretical simulation of AA-stacking (Supplementary Fig. 2).The pore size will be much smaller than 11.8 Å and can be detected by nitrogen adsorptiondesorption isotherms if it is AB-stacking.This observation (Fig. 1d), aligned with the theoretical simulation of AA-stacking (Supplementary Fig. 2), provides additional confirmation of the AA-stacking structure in the synthesized NGA-COF.

Comment 3:
How did you conduct the ICP experiment?Whether the Pt is completely dissolved is vital for the identifying of single atom density, which will determine the catalytic performance.

Response 3:
High-resolution XPS spectra of Pt 4f of working electrode (NGA-COF@Pt) before and after acid treatment.
NGA-COF@Pt was treated with 2 mL of concentrated nitric acid under ultrasound for 24 hours, leading to the complete destruction of NGA-COF and dissolution of Pt into the solution.Subsequently, 48 mL of deionized water was added, and a 10 mL sample was extracted for the ICP test.
The accuracy of Pt content determination will affect the subsequent electrochemical performance data including normalization of TOF and Mass activity.To ensure that Pt would be completely dissolved in concentrated nitric acid without remaining on the carbon paper, XPS was performed on the carbon paper which had previously loaded with NGA-COF@Pt after digestion by concentrated nitric acid.This is consistent with previous study (please refer to Fig. 2h and Table S2 of Angew. Chem.Int.Ed. 2021, 60, 19262).In addition, the error between experimental and simulated result is small enough (R factor = 1.47 % < 2 %) in the R-space fitting of EXAFS (Fig. 3e and Supplementary Table 1), indicating that the coordination environment of Pt single-atom in synthesized NGA-COF@Pt is consistent with the simulated Pt-N2 model provided inset Fig. 3e within the allowable error range.
Therefore, although the coordination environment of Pt-N is very similar to that of Pt-O, it can still be confirmed that the coordination environment of Pt in NGA-COF@Pt is Pt-N2 rather than Pt-O2 by XAS fine spectrum analysis.

Supplementary Fig. 11. (b) High-resolution XPS spectra of O 1s of NGA-COF and
NGA-COF@Pt.Also, the observed trend in the binding energy shift aligns with the theoretical calculations, as illustrated in Fig. 4a-c.All these confirm Pt-N2 rather than Pt-O2 configuration.
adjacent single-atom sites might influence each other through the interaction and migration of electron clouds.However, in our work, the density of Pt single atoms is not large enough.The distance between adjacent Pt (12.5 Å) is greater than that between metals in the M2-N4 configuration (7.4 Å).As the distance increases, the intensity of the electron interaction between adjacent Pt atoms decreases exponentially.Therefore, in this work, the interaction of the electronic structure between adjacent Pt atoms is much less than that of the metal-support interaction between Pt and NGA-COF substrate.
c.There are instances of individual adjacent Pt atoms with close distances.For instance, in Supplementary Fig. 6, two adjacent Pt atoms at a distance of 5.8 Å have been observed.However, this thermodynamically unstable configuration is almost impossible to exist either theoretically or experimentally (see the additional aberrationcorrected HAADF-STEM images of NGA-COF@Pt below) and cannot represent the overall structure of the NGA-COF@Pt catalyst.
To sum up, the more thermodynamically-stable NGA-COF@Pt (Supplementary Table 2) was finally chosen as theoretical modeling for DFT calculation to determine adsorption energy of H* intermediates and to investigate HER mechanism.This modeling comprehensively takes into account both the mathematical convergence of theoretical calculation and the realism of reaction process monitored by experiment.

Reviewer #3:
In this manuscript, the authors synthesized the first non-thermodynamically stable Pt-N2 coordination active site through a mild electrochemical modification strategy.The definite structure ensures further study of metal-support interaction and corresponding HER catalytic mechanism.Given its unique asymmetric coordination environment and single-atom characteristic, NGA-COF@Pt exhibits excellent performance and stability against poisonous H+ intermediates in acid environment.Overall, the manuscript is well written, and the work is of novelty and significance.A few minor revisions are suggested before the manuscript can be accepted for publication in Nature Communications.

Comment 1:
The authors need to explain why they prepared TiO2 NTs and used them as CE during the preparation of catalysts.The alignment issue in Fig. 2d has been fixed according to the suggestion.

Comment 4:
According to the simulative and experimental XRD and BET analysis in this manuscript, regular pore structure should be observed for this AA-stacked NGA-COF.
However, in Fig. 3a, the atomic-resolution aberration-corrected HAADF-STEM image of NGA-COF@Pt did not show micropores of ~1.2 nm size.The reason for this should be explained.

Response 4:
Additional aberration-corrected HAADF-STEM images of NGA-COF@Pt.Here are the rationales: a. Due to the numerous layers stacked in the c-axis direction of COF, a deviation of just 1° in the electron beam incidence angle can prevent the electron beam from passing through the COF and imaging on the receiver.This means that the electronic gun and the sample need to form a very precise angle, which is often difficult to achieve by manipulating the sample or filming process.
b. Unlike traditional inorganic zeolite materials, the COF material itself is not able to withstand very intense electron beam focusing.The damage to COF by electron beam is irreversible, which is likely to cause the interlayer deviation of COF, resulting in the decrease of sample crystallinity.
c.There are currently some COFs that are stable enough to be photographed with clear channel structure (J.Am.Chem.Soc.2022, 144, 12400-12409).The purpose of these works is to observe COF crystals at the atomic level.However, the synthesized NGA-COF in this paper is intended to design a conductive agent-free and pyrolysis-free platinum single atom catalyst for HER under full pH condition.So how to maintain stability in the electron beam is beyond the scope of this article.Besides, after the coordination of Pt, the structure of COF has undergone great changes compared with the original structure (Supplementary Fig. 7).Therefore, it is almost impossible to photograph such atomic-scale pore structures under aberration-corrected HAADF-STEM.
d. Since the HAADF-STEM imaging did not work as expected after many attempts, nitrogen adsorption-desorption isotherms of NGA-COF was carried out (Fig. 1d) to quantify the pore size distribution of the NGA-COF from a bulky viewpoint.It is evident that the pore size of NGA-COF is around 1.2 nm, which is in agreement with the simulations and previous literature.This approach, while unable to capture atomicscale pore structures, provides a meaningful quantification of the pore size distribution in NGA-COF from a broader perspective.

Fig. 1 c
Fig. 1 c XRD patterns of NGA-COF synthesized and simulated.

Fig. 4e
Fig. 4e High-resolution XPS spectra of N 1s of different samples.

Fig. 4a
Fig. 4a ELFs of NGA-COF and b NGA-COF@Pt with Bader charge analysis marked on specific atoms.The H, C, N, and Pt elements are shown in green, grey, blue and red, respectively.c The differential charge density distribution map of NGA-COF@Pt along the c (upper), a (bottom left) and b (bottom right) axis, where the isosurface value is set to be 0.005 e Å −3 and the positive and negative charges are shown in yellow and cyan, respectively.b.The high-resolution XPS spectra of O 1s of NGA-COF and NGA-COF@Pt was added in Supplementary Fig.11b, indicating that the chemical environments of O remained the same after the introduction of Pt.This confirms that Pt was not directly coordinated with O, ruling out the Pt-O2 coordination environment.In addition, after the introduction of Pt, there was a notable change in the binding energy of N (Fig.4e).

Response 1 :
Our previous work (Chem.Sci.2022, 13, 8876-8884) has proposed a mild electrochemical modification strategy by employing TiO2 NT as CE.Specifically, TiO2 NT CE creates a localized electric field that allows metal atoms, including Fe, Mo, Ni, Cu and Pt, to embed uniformly, precisely and robustly into the metal-organic framework (MOF)-derived carbon matrix.Previous studies have demonstrated that TiO2 NTs provide a confined electric field, resulting in more even and regular arrangement of deposited particles on the working electrode.This electrochemical modification method, previously successful with metal-organic framework-derived carbon matrices, is extended to the COF substrate in the current study.The SEM images of TiO2 NT CE used in this work are presented below: Supplementary Fig. 3. a-c SEM images of TiO2 NTs.

Comment 2 : 2 : 3 :Response 3 :
Fig. 1a Schematic illustration of the synthesis of NGA-COF.The synthesis conditions have been incorporated into Fig.1a, including solvent, catalyst, temperature, and reaction time of NGA-COF.